1. Field of the Invention
The invention involves a process for applying powder on a moving printed sheet, where powder is added into an airflow directed at the printed sheet, so that this powder is transported by the airflow to the printed sheet. The invention also involves a device for applying powder to the moving printed sheet using a first nozzle for discharging an airflow loaded with powder, which is directed at the printed sheet.
2. Related Art
A device of this type is disclosed, for example, in German patent document DE-AS 12 52 703. In that patent, several nozzle base structures are attached to a girder beam. These nozzle base structures discharge, in a fan-like manner a carrier airflow, into which an airflow loaded with powder is supplied. This is necessary since printing ink is still moist on the individual printed sheets when the printed sheets are stacked, and therefore, the printed sheets must be kept at a distance from each other using the powder. Because a spacing of individual nozzle base structures is relatively large because of a move-by gripper that grasps the printed sheet, normally between 50 to 150 mm, the carrier airflow must be adjusted in such a manner that the powder is transported with certainty to the printed sheet. Because the gripper speed is approximately 4 m/s at a printing output of 18,000 sheets per hour, relatively high cross-currents are predominant, which negatively impair the carrier airflow and thus the powder application. For example, the carrier airflow may not, under certain circumstances, spread out vertically downward as desired and thus may fail to deposit the powder as intended on the printed sheet. Thus, the carrier airflow is adjusted in such a manner that it has a relatively high air impulse current, which lies in the area of 0.04 N. In addition, the individual nozzles are arranged in such a way on each nozzle base structure that several carrier airflows of a nozzle base structure form a fan-like jet, where the individual carrier airflows relatively rapidly become unified after exiting from the nozzles into this fan-like jet. The nozzle base structures are arranged in such a way that the fan-like jets overlap each other. Therefore, the printed sheet should be relatively uniformly dusted with powder. However, it is considered disadvantageous to have fan-like jets of this type, which are relatively susceptible to cross-currents, and to have a printed sheet, which is impinged with a relatively large air impulse current. This leads to a disadvantageous effect on paper flow of the printed sheet.
It is thus an object of this invention to provide a process and/or a device, with which the printed sheet can be optimally dusted, the loss of powder is reduced and the printed sheet is impinged with a lower air impulse current.
This purpose is achieved according to the invention in a process of the type set forth at the beginning hereof, in that the airflow that carries the powder is surrounded by a powderless supporting airflow that at least partially envelops it.
A process according the invention thus provides that in addition to the airflow, which is loaded with powder, an additional airflow, namely a supporting airflow, is generated, which at least partially surrounds the powder airflow and supports and protects it in the direction to the printed sheet. The supporting airflow forms a sheath or a protective envelope around the powder airflow, so that it stays bundled on the one hand over a longer section, and on the other hand, possibly prevalent cross-currents do not act directly on the powder airflow, but instead first contact the supporting airflow. By enveloping the powder airflow with powderless air, interactions of the powder airflow with surrounding air are prevented or at least considerably reduced. Possible interfering air only entrains powderless air out of the supporting airflow, thereby ensuring no powder loss occurs.
An additional advantage is seen in that by enveloping the powder airflow, the powder airflow retains its form for a considerably longer period, so that a smaller air impulse current is necessary in order to transport the powder to the printed sheet. It has shown that in the process according to the invention, an air impulse current is required in the range from 0.01 to 0.02 N. This, however also leads to the fact that the paper flow is improved, since the printed sheet is much less loaded.
An additional embodiment provides that the supporting airflow is formed from several air jets. Instead of one ring-shaped supporting airflow, the airflow can also be formed from several, e.g. from four individual airflows, which unite immediately after exiting from additional nozzles into an enveloping jet or into several partial enveloping jets.
Advantageously, the powder airflow is supported on sides of the supporting airflow that are orthogonal to the transport direction of the printed sheet. This is advantageous because the powder airflow is protected by the supporting airflow, so that cross currents caused by the gripper only have a relatively small effect on the powder airflow.
Advantageously, the powder airflow is a discrete omnidirectional jet, as such is better suited for the transport of the powder to the printed sheet. In addition, omnidirectional jets are less susceptible to cross currents than fan jets.
The purpose of the present invention discussed heretofore is achieved using a device according to the invention in that along with a first nozzle for discharging the powder airflow, at least one additional nozzle is provided for discharging a powderless supporting airflow that at least partially envelops the powder airflow.
Thus with the device according to the present invention, two airflows are discharged which are different from each other. Via the one airflow, the powder airflow, the powder is transported to the printed sheet. The other airflow is powderless and has the functionality of supporting and protecting the powder airflow. The supporting of the powder airflow allows the airflow to retain its form over a wide range. The protective function is seen in that possible interferences through cross currents do not directly act on the powder airflow and entrain the powder, but instead possibly act on the supporting airflow, which is powderless.
In one embodiment, it is provided that to every first nozzle, four other nozzles are allocated. These four other nozzles form four partial airflows, which relatively quickly unify after they are let out of the other nozzles into the supporting airflow. Advantageously, the supporting airflow has the same flow speed as the powder airflow, so that the two currents can be fed onto each other without interference and essentially will not mix.
In order to be able to optimally oppose cross currents, the additional nozzles are arranged on the nozzle base structure essentially perpendicular to the transport direction of the printed sheet. In this manner, the powder airflow is more or less protected through an especially two-sided protected curtain.
According to the invention, the first nozzle powder airflow is constructed such that the powder airflow is a circular section jet. Circular section jets have, as already mentioned, the considerable advantage that they are relatively less susceptible to interference because of their smaller surface area and that they transport the powder optimally in the desired direction.
According to the invention, the first nozzle is constructed so that it is relatively long, so that the circular section jet can be shaped in the nozzle and keeps its shape for a relatively long time, even without the protective airflow.
Advantageously, the cross section of the first nozzle is substantially larger than the second nozzle. Since the protective airflow is not needed for the transport of the powder, but instead only in order to support and protect against interference effects, it can be constructed as a relatively thin envelope. This also has the advantage that the air impulse current, as already mentioned above, is reduced, and by this the paper flow of the printed sheet is improved. In addition, the powder airflow can, because of the protecting envelope, be supplied to the printed sheet with a smaller impulse current, where the air impulse current of the protecting current is as a rule even smaller than the powder airflow.
In one embodiment it is provided that the nozzle base structure has two first nozzles and four other nozzles allocated to each of the first nozzles. Nozzle structures with three first nozzles are also conceivable. Regardless, it is not necessary to provide nozzle structures with a plurality of nozzles in order to ensure a fan-like discharge of the powder. It is sufficient to provide two or three first nozzles, by which the powder is discharged into the powder airflow.
A special embodiment form of the nozzle structure provides that the nozzles are arranged in at least two, in particular, three planes perpendicular to the transport direction. Therefore, the other nozzles for the supporting airflow are provided in one or two planes and the nozzles for the powder airflow are provided in one plane. In this way, the prerequisite is created that the individual partial airflows form the supporting airflow relatively quickly and the two or three powder airflows, which transport the powder in the direction to the printed sheet for a relatively long time as a circular section jet, become unified with the adjacent powder airflows into a fan-like jet only shortly before the printed sheet surface.
A simple construction of the nozzle base structure is obtained according to the invention in that the nozzle is constructed in a plate-like manner in the area of the nozzle, such that one plate is provided with the first nozzle and other plates are provided with the other nozzles. This platelike construction has the advantage that the nozzle base structure can be put together in a module-like manner, and in this way can be adapted for injection molding technique.
Additional characteristics, advantages and details of the invention can be gathered from the claims as well as from the following description, in which, in reference to the drawings a particularly preferred embodiment is described in detail.